1. Field of the Invention
This invention relates to the injection system of an injection molding machine, and more particularly to an anti-backflow valve positioned at the forward end of a plasticating screw to prevent backflow of plasticated material into the screw flights upon axial movement of the screw to effect injection of the plasticated material into a mold cavity.
2. DESCRIPTION OF THE RELATED ART
In plastic molding machines utilizing a plasticating screw to convey polymeric material through a heated barrel so that it undergoes softening and is capable of flowing, the screw is rotated to carry plasticated material to the forward end of the screw. As the material accumulates ahead of the screw, the screw is permitted to move axially rearwardly until a predetermined volume of plasticated material has been accumulated ahead of the screw. At that point rotation of the screw is terminated, and the screw is caused to travel axially toward an outlet in the barrel, to thereby force the plasticated material through the outlet and inject it into an adjacent mold cavity, whereupon a part of predetermined configuration can be formed upon cooling of the material in the mold cavity.
During the injection step, while the plasticating screw is moving axially toward the outlet of the barrel, it is necessary that backflow of plasticated material from the space ahead of the screw into the screw flights be prevented, so that the desired predetermined quantity of plasticated material is completely injected into the mold cavity in order to form a complete part.
Various types of non-return or anti-backflow valve structures carried on the forward end of a plasticating screw have been devised over the years, and one example of such a valve is illustrated and described in U.S. Pat. No. 4,377,180, which issued on Mar. 22, 1983, to William H. Biljes. The non-return valve described in that patent is carried at the forward tip of the screw and includes an annular ring that is slidable a limited distance along the direction of the screw axis. When the screw is rotating and is plasticating material, the annular ring is in a forwardmost position, and plasticated material is permitted to flow through the center portion of the ring. When axial movement of the screw toward the barrel outlet commences, the annular ring contacts a sealing surface on the axially moving screw, to close off the flow path between the screw flights and the forward portion of the screw, and thereby permit injection of a desired portion of the volume of plasticated material that has been accumulated ahead of the forward end of the screw. However, the Biljes structure, which is similar to a number of other non-return valve constructions, includes a tip, around which the annular ring is carried, and that tip is secured to the forward end of the screw and rotates with the screw. It has been found that at times when the machine is first started, and plastic material is solidified form is present within the barrel, unless the plasticated material has been softened by heating before rotation of the screw is initiated, it is possible for the tip of the screw to break because the tip is surrounded by and is tightly gripped by the solidified plastic material while the screw is being rotated. Additionally, because the screw tip serves as a forward stop for the annular sealing ring during plastication, and because there is relative rotational movement between the screw tip and the sealing ring, both the screw tip and the sealing ring are required to be made from hard, wear-resistant materials in order to avoid the need for frequent replacement of those parts because of wear.
It is an object of the present invention to overcome the problems attendant with the prior art anti-backflow valve constructions, and to provide a valve structure that permits the use of less expensive materials.
It is a further object of the present invention to provide an anti-backflow valve in which the screw tip will not break on start-up of screw rotation if the plastic material surrounding the tip during a cold start is not sufficiently softened.
Other objects and advantages of the present invention will become apparent from the ensuring description.